1) Field of the Invention
The present invention relates to a thin film transistor used for a pixel circuit having a current-controlled light emitting element, and an image display apparatus.
2) Description of the Related Art
An organic electro-luminescence (EL) display apparatus using an organic light-emitting-diode (LED) that emits light autonomously is getting an attention as a next generation image display apparatus, because it does not require a back light that is necessary in a liquid crystal display (LCD) apparatus, which makes it most suitable for reducing thickness of the apparatus, and does not have any limitation in the angle of visibility. Unlike the liquid crystal display apparatus in which a liquid crystal cell is controlled by a voltage, the organic LED used for the organic EL display apparatus has a mechanism that the brightness of each light emitting element is controlled by a current.
In the organic EL display apparatus, a simple (passive) matrix type and an active matrix type can be employed as a drive system. The former has a simple configuration, but has a problem of realization of a large and high definition display. Therefore, a research and development is focused on an active-matrix-type image display apparatus that controls current flowing in a light emitting element in a pixel by a driver element such as a thin film transistor (TFT) provided in the pixel.
The pixel circuit included in the active-matrix-type image display apparatus includes a driver element that is serially connected to an organic light emitting diode (LED), and controls light emission of the organic LED and the brightness of the light emission, and a switching element that is connected to the driver element and controls the operation for transmitting voltage supplied from a data line to the inside of the pixel circuit (hereinafter, “write”) (see for example, Japanese Patent Application Laid-Open No. H8-234683 (page 10 and FIG. 1)). The voltage written into the pixel under the control of the switching element is applied to a gate electrode of the driver element, and the driver element controls the current flowing into the organic LED, regulating the current flowing through the driver element by the voltage applied to the gate electrode.
However, when the TFT using amorphous silicon for a semiconductor layer is operated for long time, the threshold voltage fluctuates, as shown in FIG. 10, and hence the current flowing through the TFT also fluctuates, causing a problem of degradation of image quality. FIG. 11 is a graph of voltage-current characteristic of a TFT with the current started to flow and the TFT with the current flowed for long time. As shown by curve I3, the threshold voltage of the initial TFT when the current starts to flow is Vth. However, the threshold voltage of the TFT in which the current has flowed for long time changes, as shown by curve I4, from Vth to Vth′ in the positive direction. At this time, even when the gate-source voltage has the same value VD, the drain current flowing through the TFT changes from Id1 to Id2 (<Id1). When the threshold voltage of the TFT used for the driver element fluctuates, even when the voltage supplied into the pixel circuit is the same, the current flowing through the driver element fluctuates, and hence the current flowing through the organic LED also fluctuates. Therefore, the emission brightness of the organic LED becomes nonuniform, thereby deteriorating the image quality of the image display apparatus.